Electroosmotic Flow Enhancement in Subnanometer Carbon Nanotubes Triggered by Water Ordering Cooperatively Induced by an Electric Field and Ion

Ion and water transport through subnanometer carbon nanotubes (CNTs) exhibits numerous anomalies absent in bulk solutions, yet the interplay among the confined water structure, external electric fields, and ion transport remains poorly understood. Here, we use systematic nonequilibrium molecular dynamics simulations to study single-file water confined in a CNT of radius 0.37 nm, both in the absence and presence of a single Na+ ion, under applied electric fields ranging from 0 to 0.2 V Å–1. In the Na+–water–CNT system, the electroosmotic flow (EOF) velocity increases nonlinearly with the field. Such nonlinear enhancement is attributed to water ordering that is exclusive to the ion-containing system. This enhanced ordering is evidenced by a larger average number of hydrogen bonds per water molecule, longer hydrogen-bond lifetimes, a reduced interoxygen distance with a higher first peak in the oxygen–oxygen radial distribution function, and a decrease in free energy. Further relative permittivity calculations reveal that the effective permittivity decreases with increasing electric field, leading to a monotonic rise in the Born energy difference between the Na+–water–CNT and water–CNT systems. This increase in Born energy likely drives the observed water ordering in Na+–water–CNT. Our findings uncover a previously unrecognized coupling mechanism among ions, electric fields, and water structure under extreme confinement.